The use of aqueous dispersions of adhesive materials is well known. Such adhesive materials can come from a variety of compound and polymer classes and can form adhesive bonds using a variety of different chemical or physical transformations. One particularly useful class of adhesive dispersions involve the use of isocyanate compounds or a polyurethane in combination with other general classes and ingredients. Such isocyanate based dispersions have been used in the past to form adhesive bonds, coatings, or other functional structures wherein post application curing is an important aspect.
One particularly important application of such adhesive dispersions is in the manufacture of laminates from sheet-like substrates. In such laminate preparation, the aqueous dispersion is applied to a sheet-like substrate forming an adhesive line or film. Solvent or water is permitted to evaporate from the adhesive film and the film is contacted with a second sheet-like substrate under typical laminating conditions of temperature, pressure or other bond forming condition to join the sheet-like substrates into a laminate structure.
In large part, the prior art adhesive dispersion use isocyanate terminated compounds or blocked isocyanate compounds. Such isocyanate adhesives use the reactivity of the isocyanate group and have been found to have a variety of processing drawbacks including unstable or difficult to make dispersion properties, light sensitivity, short pot life, etc. The isocyanate polymer dispersion materials, when held at application temperatures prior to application to a substrate material, can pre-react, separate or can tend to cause gelation in particular during adhesive manufacture. The pre-reaction can be a reaction of isocyanate groups with active hydrogens forming urea or carbamyl bonds or can be an isocyanate trimerization into isocyanurate. These isocyanate reactions are generally promoted by amine compounds such as tertiary amines which are used to stabilize the dispersion but can also act as catalysts for the undesirable trimerization reaction. Because of this pre-reaction, insufficient bond strength can result when the laminate material is formed. Further, many polymeric dispersions based on isocyanate compounds can be resistant to forming dispersions and any dispersion formed can become unstable after manufacture. The typical isocyanate polymer found in the prior art tends to have high molecular weight and high viscosity limiting its utility in aqueous dispersions.
Witt et al., U.S. Pat. No. 3,870,684 discloses a broad range of aqueous, NCO terminated, polyurethane prepolymers having acid functionality which can be reacted with chain lengthening agents including glycols and amines. The polyurethane prepolymers are derived from highly reactive aromatic and primary and secondary aliphatic isocyanates which can quickly form unmanageable gels unless the reactants and reaction conditions are carefully controlled.
Scriven et al., U.S. Pat. Nos. 4,066,591 and 4,147,679 discloses coating compositions using urethane chemistry. An NCO-- terminated prepolymer containing acid salt groups is shown with a tertiary amine to neutralize excess acid within the prepolymer. Again, careful control of reactants and reaction conditions are necessary to reduce and/or prevent the gelling of the prepolymer which is derived from highly reactive aromatic and primary and secondary aliphatic isocyanates.
Kucera et al., U.S. Pat. No. 4,540,633 discloses adhesive compositions formed by mixing a polyurethane dispersion and an epoxy dispersion.
Ramalingam, U.S. Pat. No. 4,851,459 discloses water dispersed polyurethane systems in which NCO terminated prepolymers are reacted with a peroxide compound containing at least one active hydrogen atom. These prepolymers may then be crosslinked using an epoxy functional compound.
Long, European Patent Application No. 369389 teaches NCO-- terminated aqueous dispersions containing a variety of aromatic and aliphatic isocyanate compounds in low molecular weight polymers.
"Aqueous polyurethane dispersions from TMXDI.RTM. (META) aliphatic isocyanate", American Cyanamide Company, February 1989 discloses the use of tetramethylxylene diisocyanate in the preparation of anionic dispersions of polyurethanes.
Arendt et al., "m- and p-TMXDI: Two New Isocyanates For The Polyurethane Industry", Journal of Cellular Plastics, November-December 1982 discloses the use, structure and properties of tetramethylxylene diisocyanates.